The present invention relates to a temperature-control device for samples, in particular for NMR spectroscopy, comprising a vessel provided with an opening for receiving a measuring sample, an inlet opening for introduction of a fluid, an outlet opening for the outflow of the fluid and a flow channel through which at least a partial flow of the fluid is guided past the sample, as direct fluid flow, from the bottom to the top.
A device of this type has been known, for example, from EP-A2-0 175 789.
In examining samples, for example by NMR spectroscopy, but also by optical spectroscopy or infrared spectroscopy, it is frequently required that certain sample substances be measured at different temperatures. During such measurements at different temperatures, the spectra normally vary due to the fact that, on the one hand, the heights and widths and, on the other hand, the resonance frequencies of the spectral lines may be temperature-dependent.
If this temperature-dependence of the spectra is to be determined exactly, it is necessary that a constant temperature distribution be attained over the whole active sample volume. Otherwise, adulterations of the desired effect may occur insofar as different volume elements within the sample exhibit different temperatures and may, consequently, in particular in NMR spectroscopy, produce various lines with displaced resonance points which may superpose and may, thus, lead to distorted lineshapes.
The operation of the known temperature-control devices, above all of those used in NMR spectroscopy but also of those used in optical and infrared spectroscopy, is generally such that a cooled or heated fluid, preferably nitrogen gas, is caused to flow past the surface of a cylindrical sample, from the bottom to the top, in order to bring the sample to the desired temperature.
In order to improve the spectral resolution, it is necessary in most of the cases to rotate the sample. This is effected by placing the upper area of the test tube, which contains the sample material, in the rotor of an air turbine which ensures on the one hand that the test tube is positioned correctly, and on the other hand, that it can be rotated.
However, this way of holding the sample firstly results in the situation that the upper area of the test tube cannot be sealed hermetically and that, consequently, the lower part, which can be sealed hermetically, is preferred for introducing the fluid. This means that, preferably, the fluid flow is guided past the test tube from the bottom to the top.
On the other hand, this way of holding the sample has the consequence that if the sample is heated, its bottom area will be heated to a higher temperature than its upper area as a certain amount of heat is dissipated via the point of contact between the test tube and the air turbine. It is this point of contact which plays an important part in the development of an axial temperature gradient in the test tube. In order to counteract the development of this temperature gradient, it would be desirable to have the fluid flow along the test tube from the top to the bottom; but this is connected with technical difficulties, as has been explained above.
In the case of another known arrangement, improved temperature homogeneity is achieved inside the sample by the fact that the temperature-controlling fluid is guided to the bottom of the sample by sort of a countercurrent process. The fluid flows in this case initially from the top to the bottom along the walls of a glass tube which accommodates the test tube containing the sample. The glass tube is open at its bottom so that the fluid is permitted to enter the space between the glass tube and the sample at this point from the bottom to the top and to flow thereafter along the sample from the bottom to the top, in direct contact with the test tube. If the fluid is to heat up the sample, for example, the glass tube will be heated up in this case to a higher temperature at the top than at the bottom so that, due to the radial heat transmission toward the sample, it contributes towards reducing the temperature drop encountered in the test tube in the direction from the bottom to the top. This effect, which counteracts the temperature gradient is, however, very limited because on the one hand the relatively big mass of the test tube, filled with the sample substance, requires correspondingly important heat quantities in order to change its temperature, and because on the other hand it is very difficult to transmit this relatively important heat quantity in radial direction, the wall thickness of the glass tube as well as the fluid flow between the test tube and the glass tube acting as thermal obstacles. In addition, the countercurrent principle is connected with the further drawback that the fluid flow changes its direction several times, which produces an increased flow resistance and, consequently, higher pressures for a given total flow volume .phi. of the fluid.
The conventional temperature-control device for samples illustrated in FIG. 1 shows a standard arrangement of the type used mainly in NMR spectroscopy. A sample 1 has been introduced into a vessel 20 with an outer heat insulation 3 (consisting, for example, of glass wool, expanded plastic, a vacuum, etc.), through an opening 19. A cylindrical carrier tube 6, which encloses the sample 1, carries a first RF coil 13, normally a NMR receiver coil, so that the sample is embraced by the latter. A second cylindrical carrier tube 5 comprising a second RF coil 12, for example a NMR decoupling coil, surrounds the first carrier tube 6 at a radial distance. A third wall 4, for example a cylindrical glass tube, surrounds the second carrier tube 5 at a radial distance and serves as additional thermal insulation for the arrangement.
An inlet opening 10 admits a direct fluid flow 8, normally a gas flow, which is guided against the sample 1 from below and which then enters the flow channel 18 between the sample 1 and the first carrier tube 6, leaving it again at the upper end of the sample 1 through the outlet opening 15.
By feeding the temperature-controlling fluid directly to the test tube, it is possible to attain the desired sample temperature very quickly and with a relatively small fluid flow rate. A serious disadvantage of this arrangement lies, however, in the poor axial homogeneity of the temperature in the sample, because when the sample is to be heated the test tube will get much warmer at the bottom than at the top, and when the sample is to be cooled, it will get much cooler at the bottom than at the top. This situation could of course be improved to some extent by increasing the fluid flow rate. However, since for metrological reasons the spacing between the sample 1 and the first carrier tube 6 must be kept as small as possible in order not to impair unnecessarily the signal-to-noise ratio, any increase of the fluid flow rate would lead to higher pressures in the lower area of the sample 1 and, consequently, to an inadmissible axial displacement of the test tube in upward direction.
Improved temperature homogeneity in the sample 1 is achieved by the known arrangement illustrated in FIG. 2, which operates according to the countercurrent principle. In this case, the temperature-controlling fluid is admitted to the vessel 20 through the inlet opening 10, flows through the space between the wall 4 and the second carrier tube 5 from the bottom to the top, is then deflected, by the upper cover of the vessel 20, in downward direction and into the space between the first carrier tube 6 and the second carrier tube 5, passes this space from the top to the bottom, and is then once more deflected by the lower cover of the vessel 20 in upward direction and into the flow channel 18 between the sample 1 and the first carrier tube 6. The flow channel 18 is passed by the direct fluid flow from the bottom to the top, whereafter the flow leaves the vessel 20 through the outlet opening 15.
Since in the case of this arrangement, when the sample 1 is to be heated, the direct fluid flow 8, which up to this point has dissipated practically no heat to the environment, heats up the carrier tube 6 directly neighboring the test tube more strongly in its upper area than in its lower area, the axial temperature drop necessarily occurring in the test tube in the case of the arrangement of FIG. 1 can be reduced to some extent by radial heat transmission from the first carrier tube 6 to the upper area of the sample 1. Given the fact, however, that the sample 1 has a much greater heat capacity and thermal conductivity than the other parts of the arrangement passed by the direct fluid flow 8 and that, consequently, a relatively greater amount of heat is required if the temperature gradient in the sample 1 is to be influenced notably, the radial heat transmission from the upper area of the first carrier tube 6 to the upper area of the sample 1 is by far not sufficient to attain axial homogeneity of the temperature in the sample, at least approximately. It is especially the wall thickness of the first carrier tube 6 and the direct fluid flow 8 passing between the first carrier tube 6 and the surface of the test tube which prevents any notable heat transmission. In addition, the direct fluid flow 8 is deflected several times in the case of the arrangement according to FIG. 2, and this results in a higher flow resistance which in turn leads to higher pressures for a given total flow volume .phi. of the fluid, a condition which is undesirable as well.